1. Field of the Invention
The present invention relates to an offset printing apparatus for feeding ink to a plate which is held on a plate cylinder and transferring this ink to a printing-paper which is held on an impression cylinder through a blanket cylinder, and more particularly, it relates to a printing apparatus which can selectively execute single-sided printing and double-sided printing/
2. Description of the Prior Art
For example, Japanese Patent Publication No 3-53115 (1991) describes a printing apparatus which can selectively execute single-sided printing and double-sided printing. The printing apparatus described in this gazette comprises a plurality of printing units each provided with a plate cylinder, a blanket cylinder and an impression cylinder having a diameter twice that of the plate cylinder and the blanket cylinder, and a reverse cylinder having the same diameter as the impression cylinder between each adjacent pair of printing units.
This printing apparatus is adapted to print the front surface of each printing-paper by the first printing unit, thereafter transfer the printing-paper from the impression cylinder to the reverse cylinder from the rear end of the printing-paper thereby reversing the printing-paper, and thereafter print the rear surface of the printing-paper by the next printing unit.
In case of performing four-color printing with inks of yellow (Y), magenta (M), cyan (C) and black (K), for example, four printing units are serially provided on this printing unit.
In case of performing multicolor printing in the conventional printing apparatus described in Japanese Patent Publication No. 3-53115, however, a reverse mechanism having a large-sized reverse cylinder must be provided between each pair of printing units for performing reversal printing on both sides of the printing-paper in arbitrary colors, and hence the printing apparatus is disadvantageously complicated in structure and increased in size.
Further, the conventional printing apparatus described in Japanese Patent Publication No. 3-53115 is adapted to perform printing by successively passing the printing-paper through the respective printing units. Therefore, in case of performing four-color printing by feeding inks of yellow, magenta, cyan and black to four printing units respectively and thereafter performing double-sided printing, for example, it is impossible to print both surfaces of the printing-paper with the inks of the same colors. In order to print both surfaces of the printing-paper with the inks of the same colors, therefore, the printing apparatus must perform double-sided printing after removing the inks, cleaning the printing units and feeding the same colors of inks to the plurality of printing units for exchanging the inks in the respective printing units. Also in case of performing four-color printing after printing both surfaces of the printing-paper with two colors of inks, the inks must be changed on the respective printing units.